Magnetic resonance imaging (MRI) is a widely used medical imaging technique used to investigate and diagnose the anatomy and/or physiology of the body. Typical MRI scanners use a combination magnetic fields and radiowaves to form images of the body. In general, MRI is a slow imaging method. This is partly because MRI is based on frequency space (e.g., k-space) line-scanning techniques. As such, it is inherently time-consuming to generate high-dimensional images with high spatial resolution. In most MRI exams, the operator prescribes a “slice” or “slices” in a region of interest (e.g., a kidney, a tumor, and the like) of a subject to be imaged. Because of the intrinsic properties of MRI, the slices have to be scanned line-by-line in k-space with different phase-encoding for each line.
In order to image the region of interest with a high spatial resolution, existing MRI techniques spend an excessive amount of time imaging the entire slice with equal spatial resolution. In other words, it is necessary to generate a uniform image resolution across the entire image slice. This produces an “all or nothing” approach which wastes scanning time, as the entire slice must be imaged to high spatial resolution so as to attain the desired high spatial resolution at certain region of interest. As a result, spatial resolution is often sacrificed due to practical time constraints on the scanning process, which leads to loss of fine anatomical details.
MRI data acquisition can be accelerated by partial parallel imaging (PPI) which reduces the number of k-space lines needed to form an image by taking advantage of the inherent localizing power in the heterogeneous sensitivity profiles of multi-channel receiver coils. PPI has improved clinical MRI and found widespread applications by either increasing imaging speed for the same spatial resolution, or equivalently, increasing spatial resolution for the same scan time. However, clinical PPI is limited to low imaging acceleration (i.e., <2×) due to noise amplification and convolution of artifacts at high acceleration rate. Accordingly, there is a need for MRI techniques capable of reducing the scan time necessary to image a subject at an increased focal spatial resolution without the imaging limitations suffered by PPI. If such a technique is compatible with PPI, then the increase in spatial resolution from it and that from PPI can multiply each other.